In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been, and continues to be, efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller features sizes are required. The requirement of small features with close spacing between adjacent features requires the use of high-resolution lithographic processes. In general, projection lithography refers to processes for pattern transfer between various media. In lithographic processes used for integrated circuit fabrication, a silicon wafer is coated with a radiation-sensitive film and an exposing source (such as light, X-rays, or an electron beam) illuminates selected areas of the surface of the film through an intervening master template (often referred to as a mask or a reticle) for a particular pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating causes the image area to become selectively patterned in the radiation-sensitive film and consequently either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. Further processing steps may then be performed (e.g., depositions or adding layers) based on the pattern left on the wafer.
Reticles used in exposure processes often suffer distortion in the presence of heat. When reticles are distorted, the accuracy with which some processes that utilize the reticles are performed may be compromised. Thus, the accuracy of patterning processes using reticles may be compromised. While optics may be used to compensate for some reticle distortions, some distortions may not be corrected using optics. As a result, substantially minimizing the distortion in reticles due to heat may improve the accuracy of processes performed using the reticles.
To compensate for heat-related distortion of reticles, some systems add heat to the reticles during the patterning process in an effort to maintain a substantially uniform temperature across the reticle. By evenly heating the reticles, the effect of thermal distortion of the reticles during patterning may be mitigated. However, adding heat to a reticle that is a part of a lithography system, during a patterning process may be problematic, as the addition of heat may have an adverse effect on other portions of the system. For example, the accuracy with which sensors determine positions of stages and the like may be affected, if the sensors are temperature-sensitive. Further, the addition of heat may place additional burdens on appropriate air temperature control systems.
Other systems may remove heat from the reticle by convection or conduction. By cooling the reticle, the effects of heat on the reticle also may be minimized. However, many lithography systems using Deep Ultraviolet (DUV) and Extreme Ultraviolet (EUV) are performed in a substantial vacuum, making convection cooling impractical. Conduction heating or cooling generally requires physical contact with the reticle and mechanical connections to heat sinks, heat-conductive fluids, or other conduction devices for extracting heat from the reticle. This physical contact can introduce vibrations or other displacements that comprise the accuracy of the patterning process. Moreover, use of such mechanical connections, fluids and devices may require significant changes to reticle mounting hardware.
There is a need for methods and apparatuses for cooling reticles in lithography processes that do not require physical contact with the reticle.